1. Field of the Invention
The present invention relates to a position detecting device having a cordless position indicator and its method.
2. Description of Prior Art
There is an electromagnetic transceiver system that is a method of position detecting for digitizers. This has an arrangement including, for example, a position detecting plane in which a large number of loop coils are disposed in parallel, with a position indicator such as a pen or a cursor display, wherein the loop coils are respectively used as sensors i.e. antennae, by utilizing electromagnetic interaction generated between the loop coils and the position indicator to transmit and receive electromagnetic waves between them. Based upon resulting signals detected therefrom, the system retrieves coordinate information of the indicator itself as well as other information. Through this system could be provided a principle feature. That is, that the position indicator is of a cordless system. The present applicant has proposed several digitizers in accordance with this method of electromagnetic transceiver system in Japanese Patent Gazette No. H2 (1990)- 53805, and Japanese Patent Application Laying- Open No. H3 (1991)-147012. One of the principal objects in these Patent and Patent applications is how to retrieve information from a received signal which is free from noise and at high speed.
In the method of electromagnetic transceiver system, for example, a transceiver operation is performed in which one of the antennae is sequentially selected for transmitting an electromagnetic wave and also receiving the electromagnetic wave reflected back from a resonance circuit or coil disposed within the position indicator via the selected antenna, and an arithmetic operation is performed which includes interpolation based upon the signals received via antenna which is predicted to be the strongest receiving signal and its adjacent antennas for determining the coordinate of the position indicator.
For obtaining accurate coordinate information, it is effective to place a number of antennas close to one another. But it makes the switch needed to select one of the antennae and associated control means large and complex. Therefore, it is desirable to perform detection operation with a decreased number of antennae and still with accuracy.
In the method mentioned above, the electromagnetic waves radiating back from the position indicator have very weak energy so that they cause the detection to be difficult, and if the position indicator is remotely positioned from the position detecting plane, the detection is made even more difficult. Therefore, various kinds of measures against noise are proposed by utilizing a high performance amplifier, or a low noise level signal processor, but the improvement of S/N requires a very sophisticated technique, or makes the structure more complicated and hence adds to costs.
Furthermore, applications wherein the position detecting device is equipped within the display portion of the computer, e.g. a so called "pen computer", in combination with a liquid crystal display, have come into wide use recently. When the device is used in such a high noise circumstance, more powerful anti-noise characteristics are required. Also when the device is used in combination with the liquid crystal display, the position indicator and the sensors are required to interact with each other with positioning the liquid crystal display between them since the sensors of the detecting device are disposed beneath the liquid crystal display. Therefore, it is necessary that the functionally allowable detection level of the position indicator be set higher than that of the normal position indicator. The difficulty is however increased further when the detectable level of the position detector is set higher since the signal becomes weaker in accordance with this fact. In addition, the thickness of the liquid crystal display is increasing as the recent colorization and TFT (thin film transistor) application is increasingly adapted. Accordingly, such a system is highly desired, having improved S/N performance.
Generally two groups of antennae, having the same structure to obtain two dimensional coordinate information are disposed, overlapping each other in both X and Y axes directions on the position detecting plane. For practical fabrication of these groups of antennae, the printed wiring board technique is used. Since one group of antennae is required for each of two directions (X and Y axes), a corresponding wiring pattern is provided on each layer of a two-layered printed wiring board. Each of the groups of antennae of X and Y axes has a pattern which is formed by a number of loop coils arranged in parallel on the board, each of the loop coils having folded portions, and a pattern for one axis is placed such that the folded portions of the other loop coils, which forms the pattern of the other axis, come to the inside of the former pattern. This minimizes the ineffective area, wherein no coordinate information for both X and Y axes is provided, so that the position detecting plane can be most effectively utilized. Select terminals for transmitting/receiving signals to loop coils X1, X2, . . . xn, which form a group of antennae, are shown in FIG. 2 respectively with x1, x2, . . . xn. The operation of a signal transmitting/receiving and antenna selecting circuit 102a, and a transmitting/receiving switching circuit 105a are controlled by a control circuit 101a. As shown in FIG.2, when the select terminal x6 is selected for example, the transmitting circuit 102a is firstly connected to the switching circuit 105a for sending transmission signal to loop coil x6, then the transmitting/receiving switch circuit 106a is switched to a receiving circuit 103a. As a result of the transmitting/receiving electromagnetic wave between the loop coil x6 and the position indicator, a receiving signal is generated on the loop coil x6, which is supplied to the receiving circuit 103a via the select terminal x6. Thereafter, the receiving signal is suitably processed to analyze the information included within its amplitude and phase in a discrimination circuit 104a. The transmitting/receiving operation of the respective loop coil is sequentially performed on the loop coils x1-xn.
In the above mentioned operation, it is known that the loop coil for transmitting or receiving electromagnetic wave operates more effectively as the number of turns of the coil becomes larger. Even when the current of the transmitting signal is not changed, an increment in the number of turns make an increment of strength of the transmitting electromagnetic wave in proportion to the number of turns. The same is also true with increased signal strength in receiving in proportion to the number of turn even when the strength of receiving signal is not changed. Particularly, as in the above mentioned patent application, the system in which the loop coil is commonly used as the transmitting and receiving antenna, can provide an increased efficiency both in the transmission and the reception so that it has an excellent S/N performance.
However, as shown in FIG. 2, the number of turns in each of the loop coils is practically limited to a maximum of two. The reason is that the folded portions of the loop coils forming a pattern of one coordinate is required to be located within the antenna pattern of the other coordinate as mentioned above in order to minimize the ineffective area. Referring to FIG. 2, the folded portions of loop coils X1 and X 2 are designated with 1 and 2, respectively. In this description, each interval between loop coils disposed adjacent each other, (that is, the layout interval), is designated "a", and the width of each loop coil is designated "b" as parameters which show the layouts of the loop coils.
FIG. 3 shows that the folded portions of each of the loop coils are put within the pattern for the other axis of coordinate. In FIG. 3, the patterns are designed so that the folded portions at the select terminals side of the loop coils X1, X2, . . . X14, arranged in the direction of the X axis, are all put within an interval "a1" between the loop coil Y1 and the loop coil Y2, which are disposed at the underside end among the loop coils arranged in the direction of the Y axis. In addition, the patterns are designed so that the folded portions located opposite to the select terminal side of the loop coils X1, X2, . . . X14 are all put within an interval "a2" between the loop coil Y10 and the loop coil Y11. Further, the patterns are designed so that the folded portions located opposite to the select terminal of the loop coils Y1, Y2, . . . Y11, arranged in the direction of the Y axis are all put within an interval "a3" between the loop coil X1 and the loop coil X2, which are disposed at the distal end of the loop coils arranged in the direction of the X axis. In addition, the patterns are designed so that the folded portions at the select terminal side of the loop coils Y1, Y2, . . . Y11, are all put within an interval "a4", between the loop coil X13 and the loop coil X14.
As apparent from the above description, all folded portions of the loop coils for one axis are required to be located within one of the layout intervals of the loop coils for the other axis. There also exists such a case where the folded portions of the loop coils of one axis are forced to arrange utilizing twice the portions of the layout interval of the loop coils of the other axis. In any case, it is clear that difficulty may arise in simply increasing the number of turns of loop coils since the complicated pattern design is required in order to locate every folded portion of a number of loop coils within the layout interval of a limited size. As the number of turns is increased, the number of patterns is also increased in proportion to the number of overlapped coils between adjacent loop coils. In FIG. 3, it is clear that up to 5 loop coils, such as the loop coils X1 to X5 (i.e. the number of overlaps is 5) are overlapped together, and the area where the greatest running number of folded potions exists, needs 2.times.5 wirings, in total 10 wirings. (For example, the portions designated by the numerals 100 and 110 as shown in FIG. 3). Therefore, not only the number of patterns themselves, but also the numbers of connecting potions, (such as through-holes), increased so that the number of turns are limited to 2 in prior arts.
Of course, if a multilayer printed wiring board is employed rather than a double layer printed wiring board, the number of turns can be increased further. However, in that case, the cost problem become significant, which makes the product impractical.